1. Field of Invention
The present invention relates to the gas injector of a thermal oxidation station. More particularly, the present invention relates to an injector for water free of external torch.
2. Description of Related Art
Thermal oxidation can be roughly divided into dry oxidation and wet oxidation. In dry oxidation, oxygen together with suitable amount of inert gases or nitrogen gas is passed into a heated furnace (at a temperature of around 900.degree. C.) to initiate the formation of an oxide layer on a silicon wafer. In wet oxidation, rather than passing water vapor directly into a reaction chamber, water formed by reacting gaseous hydrogen with oxygen (at an elevated temperature above 600.degree. C.) is passed into reaction chamber to carry out the oxidation. Since the water vapor formed by reacting hydrogen and oxygen is much purer and cleaner than distilled water, the silicon dioxide (SiO.sub.2) layer formed by wet oxidation has good electrical properties.
In general, the reaction between hydrogen and oxygen is carried out at high temperature inside a gas injector. A gas injector can further be distinguished by internal torch or external torch. At present, most oxidation furnace has external torch gas injector. The external torch injector comprises of a hydrogen-oxygen torch chamber and a gas injector. When gaseous hydrogen and gaseous oxygen reacts inside the torch chamber to form water moisture, the moisture is channeled into a quartz oxidation tube via a connection pipe.
FIG. 1 is a sketch showing a conventional external torch gas injector. As shown in FIG. 1, a conventional external torch gas injector 100 includes an outer tube 102 and an inner tube 104. The inner tube 104 is enclosed within the outer tube 102. The outer tube 102 has a side tube 102a and separate inner tube emission holes 102c, 102d and 102e. The outer wall of the outer tube 102 is joined to a ball-and-socket joint 108. There is an opening at the back end of the outer tube 102 for inserting a thermocouple 110. The inner tube 104 has a side tube 104a and an inner tube emission hole 104c. The side tube 102a further has an outer tube inlet 102b end the side tube 104a has an inner tube inlet 104b. The side tube 102a is roughly perpendicular to the outer tube 102 while the side tube 104a is roughly perpendicular to the inner tube 104.
To carry out wet oxidation, a quartz external torch gas injector 100 is used. Gaseous hydrogen (H.sub.2) is delivered to the inner tube inlet 104b while oxygen (O.sub.2) is passed through the outer tube inlet 102b into the hydrogen-oxygen torch chamber 112. Oxygen passes from the outer tube inlet 102b via three different outer tube emission holes 102c, 102d and 102e into the hydrogen-oxygen torch chamber 112. Hydrogen enters from the inner tube opening 104b and passes through the inner tube emission hole 104c before delivering into the hydrogen-oxygen torch chamber. Alternatively, the hydrogen can also pass through the inner tube inlet 104b into the hydrogen-oxygen torch chamber 112. During wet oxidation reaction, flow rate of oxygen into the hydrogen-oxygen torch chamber 112 must be greater than one half times that of hydrogen. In other word, the mole ratio between hydrogen and oxygen is about 1:1/2 or more to prevent hydrogen accumulation inside the chamber 112. When the concentration of hydrogen and oxygen reaches a suitable level and the temperature rises to a set level, hydrogen and oxygen will automatically ignite according to the reaction formula (a) below: ##EQU1##
Water vapor created according to formula (a) will pass out through a connection pipeline into the oxidation chamber for wet oxidation.
After the torching reaction between hydrogen and oxygen has been continued for some time, large number of water droplets 114 may accumulate near the ball-and-socket joint 108 because the temperature there is only about 60.degree. C. These accumulated water droplets 114 may affect subsequent dry oxidation operation leading to non-uniform oxide thickness.